The present invention relates to a hydrogen storage alloy electrode for use as a negative electrode of an alkaline storage battery such as nickel-metal hydride storage battery or the like.
An alkaline storage battery including a hydrogen storage alloy electrode as the negative electrode which is large in energy density and low in environmental pollution has been widely used as the power source for handy phones and electric vehicles in place of nickel-cadmium storage battery and lead acid storage battery.
Such kind of alkaline storage battery, however, has a drawback of vulnerability to corrosion of the constituents of the hydrogen storage alloy negative electrode by an alkaline electrolyte upon repeated charge and discharge cycles resulting in formation of an oxide film on the surface of the alloy. The oxide film acts to elevate the internal resistance of the battery and impairs its output characteristic. On the other hand, there has been an increasing demand recently for an alkaline storage battery having a higher output characteristic and a measure for inhibiting the corrosion of the hydrogen storage alloy has been desired to minimize decreases in the output characteristic as noted above.
Another drawback of the alkaline storage battery is that oxygen gas generating from the positive electrode upon overcharge accelerates oxidation of the hydrogen storage alloy. As a result, charge acceptance of the hydrogen storage alloy decreases and the hydrogen pressure during charge operation increases, which in turn causes an elevation of the internal pressure of the battery, a leakage of the electrolyte due to actuation of a safety valve, an increase in the internal resistance of the battery, and a decrease in discharge capacity with the progress of charge and discharge cycles.
In order to reduce the oxidation of the hydrogen storage alloy, there has been a proposed method in Japanese Laid-Open Patent Publications Hei 6-215765 and Hei 9-7588 which adds, to the hydrogen storage alloy negative electrode, a compound of yttrium such as yttrium oxide or a compound of a light rare earth element such as lanthanum, cerium, praseodymium or neodymium.
Yttrium compound such as yttrium oxide has been considered to dissolve in the alkaline electrolyte and then redeposit on the surface of the hydrogen storage alloy to form a coating on the alloy surface thereby best preventing corrosion of the alloy. However, the coating can act as a resistance during charge operation, which is not preferable from the aspect of realization of a high output power.
On the other hand, light rare earth elements such as lanthanum, cerium and the like are more basic than other rare earth elements and are liable to form a stable and passive state protective film in an alkaline electrolyte. Therefore, addition of a large amount of a compound of such light rare earth element to the electrode impairs the output characteristic of the resultant battery.